End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed for instance in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker. An echo canceller is an analog or digital device for processing a signal, such as a speech signal, so as to reduce echo by subtracting estimated echo from the echo (signal).
Normally, an echo canceller is capable of detecting a so-called disable signal. Upon detecting the disable tone, the echo canceller is switched to a "transparent" state, in which the echo canceller does not process a signal passing through. The characteristics of the disable tone are defined accurately in CCITT recommendation V.25. A disable tone is a signal of about 2100 Hz with phase inversions at intervals of (450.+-.25). The disable tone detection of the echo canceller responds to this particular signal, but not, e.g., to speech or a 2100 Hz signal with no phase inversions. Disable tones are monitored both at the receiving and transmitting end of each telephone channel. Prior art echo cancellers with disable tone detection are disclosed e.g. in U.S. Pat. No. 5,029,204.